1. Field of the Invention
This invention relates to the digital video display system, and more particularly to such a display system employing a color video encoder so as to provide composite color display.
2. Description of the Prior Art
Prior art display systems for data processing and the like were basically character displays wherein representations of characters were formed as a matrix of discrete points on the face of a cathode ray tube. Such a method of representation of characters predates electronics and may be found in embroidery, mosaics, and other disciplines employing a grid pattern. Such characters displayed on a cathode ray tube are of inferior readability unless the dot-matrix allowed for each character was much larger than 5.times.7 or 7.times.9, which are the most commonly used matrices.
Although such systems may be called video displays, they are without exception incompatible with broadcast video standards and use a standard television set or monitor in such a crude fashion that half of its spatial and all of its gray-scale resolution is lost in forming an image.
The most important reason why designers have avoided the standard broadcast type of video raster is that it is interlaced. By producing the complete frame of two interlaced fields, broadcast video achieves twice the resolution that can be achieved at that scan rate without interlace. However, dot-matrix characters appear to "flicker" when interlaced fields are used, particularly if the great majority of dots over any local region happen to fall within one or the other field. "Flicker" is a problem when interlace is employed not only for dot-matrix characters but also when there are very high contrast images. Thus, even when a standard video monitor is employed as the display device, such as for a video terminal, it is used without interlace and is limited in its vertical resolution. Because this resolution is not adequate to display a large number of good quality dot-matrix characters, some systems must employ video rasters which are incompatible with standard video monitors. It is desirable to employ standard video monitors because of their economy due to mass production.
A display system which overcomes the "flicker" problem and also successfully employs a standard monitor with interlaced scan is disclosed in the Seitz et al U.S. Pat. No. 4,158,200, which system displays characters that are made up of a plurality of different gray-scale levels or levels of luminance to create the full image of the character to be displayed on the display monitor. While the system of the Seitz et al patent can display a plurality of different gray-scale levels or luminance levels, it is not capable of displaying colored images so as to take advantage of commercially available color video sets or monitors. This invention is directed at extending the inventions described in the Seitz et al patent to color display, and in particular to the color broadcast system used in the United States (reference to National Television Standards Committee (NTSC)). Other color broadcast systems in use through the world differ from the NTSC system only in certain parameters and not in any fundamental way, so that the techniques described here are equally applicable to all major color broadcast systems.
It is evident to one skilled in the art that the basic techniques of the Seitz et al patent, in particular the use of gray-scale shading, can be applied to color images by applying the technique individually to the red, green, and blue components of the color image. However, for broadcast systems only a single signal, not three separate signals, controls the mixing of red, green, and blue. What follows particularly addresses the problem of synthesizing the single signal from a digital specification of color.
Initial attempts at digitally controlled color video systems to simulate many combinations of colors and to "colorize" gray-scale pictures resulted in images which appear to "glimmer". That is to say, if a picture with high frequency video components was colorized, a crawling effect appeared along the edge where abrupt changes in color occurred. This effect was caused by a lack of bandwidth limiting of the chrominance signal.
This could be corrected in part by encoding the luminance information separately from the chrominance information and employing digital filters. However, the filters cause a delay in the generation of the color information with respect to the luminance information and the large area of the analog section of the system was devoted to adjustments. Furthermore, the color filtering was not to standards accepted by the Federal Communications Commission (FCC) for the compatible, composite, color video waveform.
It is, then, an object of the present invention to provide an improved digital display system for the color display of characters, images, and other information.
It is another object of the present invention to provide an improved digitally controlled color video display system for pictures with high frequency video components without incurring "glimmer" effects.
It is still another object of the present invention to provide a display system that employs digital control for standard commercial video monitors.